In most resin processing apparatus, particularly injection molding and extruding machines, moisture absorbed by or otherwise entrained in the granular or pelletized resins supplied to the processing apparatus presents substantial difficulties. Many resin materials are hygroscopic, or at least may include appreciable quantities of entrained moisture. For high quality molded plastics or extrusions, however, a low moisture content is usually required. For example, some resinous materials may normally have a water content of over 0.2% whereas the processing apparatus may require resin with a water content of 0.08% or less to produce a quality product.
A thermal dryer for reducing the moisture content of such granular resins is disclosed in Waters U.S. Pat. No. 3,875,683. In that dryer the granular resin passes downwardly through a plenum chamber in which it is contacted with a drying gas, usually air. The drying air is heated to a predetermined temperature immediately prior to contact with the granular resin. This type of thermal dryer, while performing well in most respects, is rather inefficient in regard to heat loss characteristics, and temperature control over the drying air may be difficult to maintain.
Another thermal drying apparatus, employed to dry granular resin immediately prior to its introduction into an extruder or injection molding machine, is described in Nielson et al U.S. Pat. No. 4,531,308. The Nielson et al apparatus is generally similar to that described in the Waters patent but is usually somewhat smaller in size and is mounted directly on an extruder, injection molding machine, or other resin processing apparatus. This form of thermal dryer also is rather inefficient in its utilization of the energy required to heat the drying air. Precision control of the temperature of the drying air, as applied to the granular resin, can be difficult to realize. Further, the dryer tends to interfere with some operations of the resin processing apparatus, particularly on a changeover from one product to another.